IMO, the “red line” for 1-cast-clearing is likely set at E9.
An interesting observation is that the level increase between Explore difficulties doubles between E9 an E10 (a 20 level gap). I think that this is more of a moat that was dug to insulate E10 from being 1-cast farmable, even well into the future possibly even forever.
But… power creep, though?
Well, the outlook is rather gloomy on that. Right now, players are enjoying some significant leaps in power, because of kingdoms reaching Power 20 and Level 15, somewhat hand-in-hand. After that though… in terms of stat growth, the future is as bleak as the Drifting Sands.
As of now, all magic kingdoms except Silverglade and Zhul’Kari have already obtained their Power 20 stat boosts, so there is only +2 magic left to be obtained here.
The Underworld only has +1 magic left to give for perfect completion one all factions there have been released.
Assuming that the third map will play out similar to the Underworld as another map with factions on it (all signs and tea leaves continue to strongly point to this outcome), that’s another +2 potential magic in ~3 years’ time.
Power 30 and Level 20 for kingdoms is similarly at least ~3 years away from occurring (likely loosely tied in timing to completion of releases for the third map) for the next set of kingdom stat boosts.
Outside of some new stat mechanic or booster being released in the future, once a end-gamer has all kingdoms at Power 20 and Level 15 (within the next year easily), stat growth is going to reach almost a complete standstill for as far as the eye can see.
Unless one of remaining yet-to-be-revealed kingdoms is a magic kingdom, I’m rather dubious on E10 ever being one-shottable. And, as others have already said, if a player is in two-cast territory, they might as well be two-shotting E12.
EDIT: Thanks to @Grundulum and @Raistlin1 for noting that Nysha tokens can technically drop from E9. Erroneous paragraph was removed.
EDIT 2: Thanks to @Jefferson for correctly pointing out that the magic point at account rank 2000 only applies to the hero(ine) and is not a global magic stat point. Removed that bullet point from the post.